Local structures and photocatalytic reactivities of the titanium oxide and chromium oxide species incorporated within micro- and mesoporous zeolite materials: XAFS and photoluminescence

Current Opinion in Solid State and Materials Science 7 (2003) 471–481

Local structures and photocatalytic reactivities of the titaniumoxide and chromium oxide species incorporated within micro- andmesoporous zeolite materials: XAFS and photoluminescence studies

Hiromi Yamashita a,*, Masakazu Anpo b

a Department of Materials Science and Processing, Graduate School of Engineering, Osaka University, Yamada-oka 2-1, Suita,

Osaka 565-0871, Japanb Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Gakuen-cho 1-1, Sakai,

Osaka 599-8531, Japan

Received 28 February 2004; accepted 28 February 2004

Abstract

Transition metal oxides (titanium, chromium oxides) photocatalysts can be designed within the cavities and frameworks of

various zeolites and mesoporous molecular sieves by ion-exchange and hydrothermal synthesis. A combination of in situ XAFS and

photoluminescence spectroscopic techniques has revealed that these transition metal oxide species incorporated within zeolites exist

in a highly dispersed state with a tetrahedral coordination and such a high dispersion state allows them to act as efficient photo-

catalysts for various photocatalytic reactions. These results clearly suggest that the utilization of zeolites and mesoporous materials

is one of the most promising approaches to design and development of highly efficient and effective photocatalysts with well-defined

local structures at the molecular level.

� 2004 Published by Elsevier Ltd.

Keywords: Photocatalyst; Zeolite; Mesoporous molecular sieve; XAFS; Photoluminescence; Titanium oxide; Chromium oxide

1. Introduction

There is special interest in designing the ion and/or

cluster size catalysts within zeolites (microporous zeolite

and mesoporous molecular sieve materials) [1–3,**4,5–8]

because these fascinating supports offer unique nano-

scaled pore systems, unusual internal surface topology,and ion-exchange capacities. With catalysts anchored

within zeolites it becomes possible to design active sur-

face species which span the range from discrete molecules

to aggregated clusters, and finally to extended bulk

semiconducting materials. In addition, zeolites with well-

defined nano-pore structure provide one of the most

promising modified spaces for photocatalytic reactions.

The unique and fascinating properties of zeolitesinvolving transition metals within the zeolite cavities and

framework have opened new possibilities for many

application areas not only in catalysis but also for various

*Corresponding author. Tel.: +81-6-6879-7457.

E-mail address: [email protected] (H. Yamashita).

1359-0286/$ - see front matter � 2004 Published by Elsevier Ltd.

doi:10.1016/j.cossms.2004.02.003

photochemical processes [9–13,*14,15–17,*18,19,20,*21,

22–26,**27,28,*29,30,*31,32,*33,34–37,**38,39]. Tran-

sition metal ions in metallosilicate catalysts are consid-

ered to be highly dispersed at the atomic level and also to

be well-defined catalysts which exist in the specific

structure of the zeolite framework [1–3,**4,5–13,

*14,15,16]. According to the L€owenstein rules [8], a well-prepared zeolite sample should contain only the iso-

lated metal ions. Furthermore the counter cations in

zeolites are very easily exchanged through different ca-

tions by conventional ion-exchange methods. Since the

exchangeable sites are separated from each other within

the zeolite cavities under well controlled conditions,

ion-exchange with metal ions having photocatalytic

capabilities can be used for the preparation of uniquephotocatalysts. These properties of zeolites are of great

significance in the design of highly dispersed transition

metal oxide catalysts such as Ti, V, Cr, Mo, etc., which

can be excited under UV-irradiation by the following

charge transfer process (Scheme 1).

These charge transfer excited states, i.e., the electron–

hole pair state which localize quite near to each other as

mail to: [email protected]

hv[ Mn+ O2- ] [ M(n-1)+ O- ]*

(M: Ti, V, Cr, Mo ....)

Scheme 1. The formation of charge transfer excited state with transi-

tion metal oxide moieties by light absorption.

472 H. Yamashita, M. Anpo / Current Opinion in Solid State and Materials Science 7 (2003) 471–481

compared to the electron and hole produced in semi-

conducting materials, play a significant role in various

photocatalytic reactions such as the photo-reduction of

CO2 with H2O [28,*29,30,*31,32], the decomposition of

NO into N2 and O2 [*33,34,35] or the degradation

of organic impurities in water [40–42], the photo-oxi-

dation reaction of hydrocarbons [16,**38,39] and thephotoinduced metathesis reaction of alkanes [43]. These

photocatalytic reactions were found to proceed with

high efficiency and selectivity, displaying quite different

reaction mechanisms from those observed on semicon-

ducting TiO2 photocatalysts in which the photoelectro-

chemical reaction mechanism plays an important role.

Recently unique and efficient photocatalytic systems

of the transition metal oxides (Ti, V, Cr, etc.) have beendesigned and developed using the cavities and frame-

work of zeolites and mesoporous molecular sieves [9–

13,*14,15–17,*18,19,20,*21,22–26,**27,28,*29,30,*31,

32,*33,34–37,**38,39]. These photocatalyst systems

incorporated within the zeolite cavities and framework

have been found to be effective for various photocata-

lytic reactions [1–3,**4,5–8]. In this review, the prepa-

ration of the photocatalysts, the characterization of theactive sites and their dynamics as well as a clarification

of the mechanisms behind the observed photocatalytic

reactions have been discussed. The XAFS and photo-

luminescence measurements in particular are very useful

in the characterizing the active sites and their dynamics

at the atomic level. With the useful information ob-

tained with these powerful techniques, special attention

has been focused on the relationship between the localstructures of the active sites for the photocatalytic

reactions and their photocatalytic properties.

Fig. 1. XANES (left) and FT-EXAFS (right) spectra of anatase TiO2

powder (a, a0), the imp-Ti-oxide/Y-zeolite (10 wt.% as TiO2) (b, b0), the

imp-Ti-oxide/Y-zeolite (1.0 wt.% as TiO2) (c, c0), the ex-Ti-oxide/Y-

zeolite (d, d0), and the Pt-loaded ex-Ti-oxide/Y-zeolite (e, e0) catalysts.

2. Titanium oxides (Ti-oxide/Y-zeolite, Ti-mesoporous

molecular sieve)

2.1. Photocatalytic reduction of CO2 with H2O

The photocatalytic reduction of CO2 with H2O is of

interest, not only as a reaction system utilizing artificialphotosynthesis, but also as a way to use carbon sources

for synthesis of hydrocarbons and oxygenates such as

CH4 and CH3OH [28,*29,30,*31,32,44,45]. The photo-

catalytic reactivity of a highly dispersed titanium oxide

catalyst anchored onto porous silica glass or zeolites was

also investigated and it was found that the highly dis-

persed titanium oxide catalyst exhibits a high and un-

ique photocatalytic reactivity as compared to bulk TiO2

powder [28,*29,30,*31,32,44]. These results indicate that

by using zeolites as supports, highly dispersed titanium

oxides can be produced, leading to the development ofenvironmentally-friendly photocatalytic systems having

high catalytic efficiency, selectivity and other fascinating

properties such as shape selectivity and a reactant gas

condensation effect.

The Ti-oxide anchored onto zeolite, Ti-oxide/Y-zeo-

lite (1.1 wt.% as TiO2), was prepared by ion-exchange

with an aqueous titanium ammonium oxalate solution

using Y-zeolite (SiO2/Al2O3 ¼ 5.5) (ex-Ti-Oxide/Y-zeo-lite). Ti-oxide/Y-zeolites having different Ti contents (1.0

and 10 wt.% as TiO2) were prepared by impregnating

the Y-zeolite with an aqueous solution of titanium

ammonium oxalate (imp-Ti-oxide/Y-zeolite). Ti-MCM-

41, Ti-MCM-48 and Ti-HMS (Si/Ti¼ 50–100) were

hydrothermally synthesized or synthesized at ambiguous

conditions [5,28,*29,30,*31,32,*33,46,47].

2.1.1. Ti-oxide anchored on zeolite prepared by the ion-

exchange

Fig. 1 shows the XANES and the Fourier transfor-

mation of EXAFS (FT-EXAFS) spectra of the Ti-oxide/

Scheme 2. The formation of charge transfer excited state with tetra-

hedrally coordinated titanium oxide moieties by UV light absorption

and their radiative decay process (phosphorescence).

H. Yamashita, M. Anpo / Current Opinion in Solid State and Materials Science 7 (2003) 471–481 473

Y-zeolites. The XANES spectra of the Ti-oxide catalysts

at the Ti K-edge show several well-defined preedge peaks

which are related to the local structures surrounding

the Ti atom [25,26,**27,48]. The ex-Ti-oxide/Y-zeoliteexhibits an intense single preedge peak indicating that

the Ti-oxide species have a tetrahedral coordination

[*29,*33]. On the other hand, the imp-Ti-oxide/Y-zeolite

prepared by the impregnation exhibits three character-

istic weak preedge peaks attributed to crystalline anatase

TiO2. The FT-EXAFS spectra of the ex-Ti-oxide/Y-

zeolite exhibits only peak assigned to the neighboring

oxygen atoms (Ti–O) indicating the presence of an iso-lated Ti-oxide species. These findings indicate that highly

dispersed isolated tetrahedral Ti-oxide species are

formed on the ex-Ti-oxide/Y-zeolite. On the other hand,

the imp-Ti-oxide/Y-zeolite exhibits an intense peak as-

signed to the neighboring titanium atoms (Ti–O–Ti),

indicating the aggregation of the Ti-oxide species.

Fig. 2 shows a typical photoluminescence spectrum of

the Ti-oxide anchored onto zeolite (ex-Ti-oxide/Y-zeo-lite) at 77 K. Excitation by light at around 230–270 nm

brought about an electron transfer from the oxygen to

titanium ion, resulting in the formation of pairs of

the trapped hole center (O�) and an electron center

(Ti3þ) [**4,5–8,28,*29,30,*31,32,*33,34–37,**38,39–42].

The observed photoluminescence is attributed to the

radiative decay process from the charge transfer excited

state of the Ti-oxide moieties having a tetrahedral coor-dination, (Ti3þ–O�)�, to their ground state [**4,5–8,

28,*29,30,*31,32,*33,34–37,**38,39–42] as shown in

Scheme 2. As shown in Fig. 2 the addition of H2O or CO2

molecules onto the anchored Ti-oxide species leads to the

efficient quenching of the photoluminescence. Such an

Fig. 2. The observed ordinary photoluminescence spectrum of the ex-

Ti-oxide/Y-zeolite catalyst (a), its excitation spectrum (EX), and effects

of the addition of CO2 and H2O (b, c) and the loading of Pt (d) on the

photoluminescence spectrum at 77 K. Excitation at 270 nm, emission

monitored at 490 nm, amounts of added CO2: (b) 8.5, and H2O; (c) 2.9

lmol g�1.

efficient quenching suggests not only that tetrahedrally

coordinated Ti-oxide species locate at positions accessi-

ble to the added CO2 or H2O but also that added CO2 or

H2O interacts and/or reacts with the Ti-oxide species in

both its ground and excited states. Because the addition

of CO2 led to a less effective quenching than with theaddition of H2O, the interaction of the emitting sites with

CO2 was weaker than with H2O.

UV-irradiation of powdered TiO2 and Ti-oxide/Y-

zeolite catalysts in the presence of a mixture of CO2 and

H2O led to the evolution of CH4 and CH3OH in the gas

phase at 328 K, as well as trace amounts of CO, C2H4

and C2H6 [28,*29,30,*31,32,44,45]. The yields of these

photoformed products increased linearly against theUV-irradiation time, indicating the photocatalytic

reduction of CO2 with H2O on the catalysts. The specific

photocatalytic reactivities for the formation of CH4 and

CH3OH are shown in Fig. 3. The ex-Ti-oxide/Y-zeolite

exhibits a high reactivity and a high selectivity for the

formation of CH3OH while the formation of CH4 was

found to be the major reaction on bulk TiO2 as well as

on the imp-Ti-oxide/Y-zeolite. These findings clearlysuggest that the tetrahedrally coordinated Ti-oxide

species act as active photocatalysts for the reduction of

Fig. 3. The product distribution of the photocatalytic reduction of

CO2 with H2O on anatase TiO2 powder (a), the imp-Ti-oxide/Y-zeolite

(10 wt.% as TiO2) (b), the imp-Ti-oxide/Y-zeolite (1.0 wt.% as TiO2)

(c), the ex-Ti-oxide/Y-zeolite (1.1 wt.% as TiO2) (d) and the Pt-loaded

ex-Ti-oxide/Y-zeolite (e) catalysts.


CO2 with H2O showing a high selectivity to produce

CH3OH.

2.1.2. Ti-mesoporous molecular sieve

The Ti-oxide species prepared within the framework

of zeolites have revealed a unique local structure as well

as a high selectivity in the oxidation of organic sub-

stances with hydrogen peroxide [2,3]. Ti-containingzeolites (TS-1, Ti-b) and mesoporous molecular sieves

(Ti-MCM, Ti-HMS, Ti-HMS) have been synthesized

[5,28,*29,30,*31,32,46,47] and can be utilized as efficient

photocatalysts.

In situ photoluminescence, ESR, UV–VIS and XAFS

investigations indicated that the Ti-oxide species in the

Ti-mesoporous molecular sieves (Ti-MCM-41 and Ti-

MCM-48) and in the TS-1 zeolite are highly dispersedwithin the zeolite framework and exist in a tetrahedral

coordination. Upon excitation with UV light at around

250–280 nm, these catalysts exhibit photoluminescence

spectra at around 480 nm. The addition of CO2 or H2O

onto these catalysts results in a significant quenching of

the photoluminescence, suggesting the excellent acces-

sibility of the Ti-oxide species to CO2 and H2O

[5,28,*29,30,*31,32].UV-irradiation of the Ti-mesoporous molecular

sieves and the TS-1 zeolite in the presence of CO2 and

H2O also led to the formation of CH3OH and CH4 as

the main products (18–21). The yields of CH3OH and

CH4 per unit weight of the Ti-based catalysts are shown

in Fig. 4. It can be seen that Ti-MCM-48 exhibits much

higher reactivity than either TS-1 or Ti-MCM-41. Be-

sides the higher dispersion state of the Ti-oxide species,other distinguishing features of these zeolite catalysts

are: TS-1 has a smaller pore size (�5.7 �A) and a three-

dimensional channel structure; Ti-MCM-41 has a large

pore size (>20 �A) but one-dimensional channel struc-


CO2 with H2O on anatase TiO2 powder (a), TS-1 (b), Ti-MCM-41 (c),

Ti-MCM-48 (d) and the Pt-loaded Ti-MCM-48 (e) catalysts.

ture; and Ti-MCM-48 has both a large pore size (>20 �A)

and three-dimensional channels. Thus, the higher reac-

tivity and higher selectivity for the formation of CH3OH

observed with the Ti-MCM-48 than with the othercatalysts may be due to the combined contribution of

the high dispersion state of the Ti-oxide species and the

large pore size with a three-dimensional channel struc-

ture. These results strongly indicate that mesoporous

molecular sieves with highly dispersed Ti-oxide species

are promising candidates as effective photocatalysts.

Furthermore, the photocatalytic reduction of CO2

with H2O on Ti-containing mesoporous silica (Ti-HMS)with various levels of titanium content has been inves-

tigated. Ti-HMS with various titanium contents were

synthesized by using TEOS, TPOT, and dodecylamine

as the structure direction agent [35,46]. XAFS and UV–

VIS absorption studies indicated that Ti-HMS with

lower Ti content included the tetrahedrally coordinated

Ti-oxide species which could exhibit the photolumines-

cence under UV irradiation. UV irradiation of Ti-HMSin the presence of a mixture of H2O and CO2 led to the

formation of CH4, CH3OH and CO as well as trace

amounts of C2H4 and O2, showing good linearity

against irradiation time. Fig. 5 shows the yields of CH4

and CH3OH in the photocatalytic reduction of CO2 with

H2O on Ti-HMS with various levels of Ti content. The

Ti-HMS with the lower Ti content exhibits the higher

reactivity for formation of CH4 and CH3OH and thephotocatalytic reactivity has a strong relation with

photoluminescence intensity. These results also indicate

that the charge transfer excited state of Ti-oxide species

play an important role in photocatalytic reduction of

CO2 with H2O to produce CH3OH with high selectivity.

2.1.3. Effect of Pt-loading

The effect of Pt-loading on the photocatalytic reac-

tivity of Ti-containing zeolite has also been investigated

Fig. 5. The relationship between yields of the photoluminescence and

the yields of CH4 and CH3OH in the photocatalytic reduction of CO2

with H2O on the Ti-HMS catalysts with various Ti contents.


CO2 with H2O on Ti-b(F), Ti-b(OH), and TiO2 powder (P-25) as the

reference catalyst.

4965 4970 4975

Nor

mal

ized

abs

orpt

ion

/ a. u

.

(a)

(b)

Fig. 7. The effect of the addition of H2O molecules on the intensity

and position of the preedge peak observed in the Ti K-edge XANES

spectra of Ti-b(OH) (a) and Ti-b(F) (b) zeolites. The amount of the

added H2O molecules; 0, 1.4, 3.0, 4.6 mmol/g cat (from top to bottom).


(Fig. 3). Although the addition of Pt onto the Ti-con-

taining zeolites is effective for an increase in the photo-

catalytic reactivity, only the formation of CH4 is

promoted [*29]. As shown in Fig. 1, the Pt-loaded cat-alyst also exhibits the same preedge peak in the XANES

spectra and the same Ti–O bonding peak in the FT-

EXAFS spectra as those of the original Ti-containing

zeolite. Furthermore, as shown in Fig. 2, Pt-loading

onto the Ti-containing zeolite catalyst leads to an effi-

cient quenching of the photoluminescence, accompanied

by the shortening of its lifetime. The effective quenching

of the photoluminescence can be attributed to the elec-tron transfer from the photoexcited Ti-oxide species to

Pt metals which exist in the neighborhood of the Ti-

oxide species. The electrons are easily transferred from

the charge transfer excited state of the Ti-oxide species,

the electron–hole pair state of (Ti3þ–O�)�, to the Pt

moieties. As result, on the Pt-loaded Ti-containing

zeolite catalyst, photocatalytic reactions which proceed

in the same manner as on bulk TiO2 catalysts becomepredominant, and the reduction reaction by electrons

and the oxidation reaction by holes occur separately

from each other on different sites, leading to the selective

formation of CH4.

2.1.4. Effect of surface hydrophilic–hydrophobic proper-

ties

Recently a large-pore Ti-containing zeolite, Ti-b, hasbeen hydrothermally synthesized (35–37). The H2O

affinity of Ti-b zeolites changes significantly depending

on the preparation methods and their hydrophobic–

hydrophilic properties can modify the catalytic proper-

ties [*31,32]. As shown in Fig. 6, the photocatalyticreduction of CO2 with H2O to produce CH4 and

CH3OH was found to proceed in the gas phase at 323 K

with different reactivities and selectivities on hydrophilic

Ti-b(OH) and hydrophobic Ti-b(F) zeolites prepared in

the OH- and F-media, respectively. The higher reactivity

for the formation of CH4 observed with Ti-b(OH) and

the higher selectivity for the formation of CH3OH ob-

served with the Ti-b(F) may be attributed to the differentabilities of zeolite pores on the H2O affinity. These re-

sults suggest that the hydrophilic-hydrophobic property

of surface of zeolite cavities is one of the important

factors for selectivity in the photocatalytic reduction of

CO2 and H2O.

The advanced applications of in situ XAFS mea-

surements of Ti-containing zeolites were made by the

research group of Thomas and Sankar [25,26,**27]. Inthe present study, the interaction of titanium oxide

species incorporated within the zeolite framework with

H2O and CO2 molecules has been investigated by in situ

XAFS measurement. As mentioned above, the change in

the coordination geometry of Ti atom reflects very

sensitively on the intensity and position of preedge peak

in XANES region at Ti K-edge [25,26,**27,48]. Fig. 7

shows the preedge peak in the XANES spectra of Ti-

b(OH) and Ti-b(F) zeolites and the effect of the addition

of H2O molecules on their preedge peaks. The intensity

and position of the preedge peaks clearly indicate the

presence of the tetrahedrally coordinated titanium oxide

species in these zeolites under vacuum condition. As

shown in Fig. 6, the addition of H2O molecules onto theTi-b zeolites leads to the efficient decrease in the peak

intensity and the shift to the higher energy in the peak


position, its extent depending on the amount of added

H2O. Such changes suggest not only that tetrahedrally

coordinated titanium oxide species locate at positions

accessible to the added H2O molecules but also thatadded H2O molecules interact directly with the titanium

oxide species changing its coordination geometry.

The change in the intensity and position of preedge

with the addition of H2O molecules also shown in Fig. 8.

From the changes of these values with the H2O addition,

it is clearly found that the coordination number of

titanium oxide species increases from its original four-

coordination to five-coordination and finally to six-coordination. As shown in Figs. 7 and 8, the changes in

the peak intensity and position with the H2O addition

are more remarkable on Ti-b(OH) than Ti-b(F). Thisdifference indicates that the opportunity for interaction

between the added H2O molecules and titanium oxide

species in the zeolite framework is the higher in the pore

of hydrophilic Ti-b(OH) than the hydrophobic Ti-b(F).The hydrophobic–hydrophilic properties of the zeoliteaffect the accessibility and the interaction between

photocatalytic active sites (tetrahedrally coordinated

titanium oxide species) and reactant gasses (H2O mole-

cules) and finally become the important factor in

determining the selectivity in the formation of CH3OH.

2.2. Photocatalytic decomposition of NO on Ti-oxide

included within zeolites

UV-irradiation of the powdered TiO2 and the Ti-

oxide highly dispersed on zeolite cavities and frame-

Fig. 8. Plots of normalized height vs energy of the Ti preedge feature

showing the values observed with Ti-b(OH) (A–D) and Ti-b(F) (a–d)zeolites in the absence and presence of added H2O molecules as well as

the areas for four-, five-, and six-coordinated Ti for well-characterized

Ti-model compounds reported previously by Thomas et al. [**27]. The

amount of the added H2O molecules; (A, a) 0, (B, b) 1.4, (C, c) 3.0, (D,

d) 4.6 mmol/g cat.

works in the presence of NO were found to lead to the

evolution of N2, O2 and N2O in the gas phase at 275 K

with different yields and different product selectivities

[*33,34,35]. The efficiency and selectivity for the for-mation of N2 strongly depend on the type of catalysts.

The Ti-oxide highly dispersed on zeolite cavities and

frameworks exhibits a high reactivity and a high selec-

tivity for the formation of N2 while the formation of

N2O was found to be the major reaction on the bulk

TiO2 catalyst as well as on the Ti-oxide/zeolite prepared

by the impregnation.

XAFS investigations of Ti-oxide catalysts at the TiK-edge were performed and the results revealed that the

titanium oxide species has a tetrahedral coordination in

the case of the Ti-oxide highly dispersed on zeolite

cavities and frameworks while the titanium oxide species

has a octahedral coordination in the case of the catalyst

prepared by the impregnation. Fig. 9 shows the rela-

tionship between the coordination number of titanium

oxide species revealed by XAFS studies and the selec-tivity for N2 formation in the photocatalytic decompo-

sition of NO on various titanium photocatalysts. The

clear dependence of the N2 selectivity on the coordina-

tion number of the titanium oxide species can be

observed, i.e., the lower the coordination number of tita-

nium oxide species, the higher the N2 selectivity. From

these results, it is proposed that a highly efficient and

selective photocatalytic reduction of NO into N2 and O2

can be achieved using the Ti-oxide/zeolite as a photo-

catalyst which includes the highly dispersed isolated

tetrahedral titanium oxide as the active species, while the

formation of N2O as the major product was observed

for the bulk TiO2 catalysts and on the catalysts which

include the aggregated octahedrally coordinated tita-

nium oxide species.

Sel

ectiv

ity fo

r N

2 F

orm

atio

n / %

Coordination Number

Ti4+

O2-O2- O2-

O2-

O2- O2-

Ti4+

O2-O2- O2-

O2-

65.554.543.5 6.50

100

80

60

40

20

Fig. 9. The reaction time profiles of the photocatalytic decomposition

of NO into N2 and N2O on the ex-Ti-oxide/Y-zeolite.


3. Chromium oxide (Cr-mesoporous molecular sieve)

Highly dispersed Mo or Cr-oxides catalyst have been

shown to exhibit high activities for various photocata-lytic reactions such as the photo-oxidation reaction of

hydrocarbons or the photoinduced metathesis reaction

of alkanes [16,43,49]. Recently, it was found that the Cr-

containing mesoporous zeolite (Cr-HMS) shows pho-

tocatalytic activities for the propane oxidation and NO

decomposition either under UV or even visible light

irradiation [36,37,**38,39].

Cr-containing mesoporous molecular sieves (Cr-HMS) (0.2, 2.0 wt.% as Cr) were synthesized using tetra-

ethylorthosilicate, Cr(NO3) Æ 9H2O as the starting

materials and dodecylamine (DDA) as a template

[36,37,**38,39]. Cr-silicalite (CrS-1) microporous zeo-

lites (0.2 wt.% as Cr) were hydrothermally synthesized

using tetrapropyl ammonium hydroxide (TPAOH) as a

template [36,37,**38,39]. Imp-Cr/HMS (Si/Cr¼ 100)

was prepared by impregnating HMS with an aqueoussolution of Cr(NO3)3 Æ 9H2O.

Fig. 10 shows the XAFS (XANES and FT-EXAFS)

spectra of the treated Cr-HMS and imp-Cr/HMS. Cr-

HMS exhibits a sharp and intense preedge peak which is

Fig. 10. Cr K-edge XANES spectra (A–D) and Fourier transforms of

EXAFS spectra (a–d). (a) CrO3, (b) Cr2O3, (c) Cr-HMS (Si/Cr¼ 50),

(d) imp-CrHMS (Si/Cr¼ 50). R: atomic distance (�A), N: coordination

number.

characteristic of Cr-oxide moieties in tetrahedral coor-

dination having terminal Cr@O. In the FT-EXAFS

spectrum, only a single peak due to the neighboring

oxygen atoms (Cr–O) can be observed showing that Crions are highly dispersed in Cr-HMS [36,37,**38,39,50].

The imp-Cr/HMS exhibits a weak preedge peak in the

XANES spectra and an intense peak due to the neigh-

boring Cr atoms (Cr–O–Cr) in the FT-EXAFS spectra,

indicating that the catalyst consists of a mixture of tet-

rahedrally and octahedrally coordinated Cr-oxide spe-

cies (Cr2O3-like cluster).

As shown in Fig. 11, the UV–VIS spectra of the Cr-HMS catalysts exhibit three distinct absorption bands at

around 250, 350 and 450 nm which are assigned to the

charge transfer from O2� to Cr6þ of the tetrahedrally

coordinated Cr-oxide species. The absorption bands

assigned to the absorption of the dichromate or Cr2O3

cluster cannot be observed above 550 nm, indicating

that tetrahedrally coordinated Cr-oxide species exists in

an isolated state.As shown in Fig. 12, Cr-HMS catalysts exhibit pho-

toluminescence spectra at around 550–750 nm upon

excitation of the absorption (excitation) band at around

450 nm. These absorption and photoluminescence spec-

tra are similar to those obtained with well-defined highly

dispersed Cr-oxides anchored onto Vycor glass or silica

[36,37,**38,39,49,51] and can be attributed to the charge

transfer processes on the tetrahedrally coordinated Cr-oxide species involving an electron transfer from O2� to

Cr6þ and a reverse radiative decay, respectively. These

results indicate that the Cr-HMS mesoporous zeolite

involves Cr-oxide moieties in tetrahedral coordination

having terminal Cr@O, being in good agreement with

the results obtained by XAFS and UV–VIS measure-

ments. The estimated model for the local structure of the

Cr-oxide moieties and the charge transfer excited stateare shown in the Scheme 3.

In the photoluminescence of the Cr/HMS, fine

structures can be observed which are due to the

Fig. 11. Diffuse reflectance UV–VIS spectra of Cr-HMS (A–D) and

HMS (D). (A) Si/Cr¼ 50, (B) Si/Cr¼ 100, (C) Si/Cr¼ 500.

500 600 800Wavelength / nm

Inte

nsity

/ a.

u.

700

Ex: 520nm 0Torr

0.30

0.71

1.42

2.99

12.40

vac.

500 600 700 800Wavelength / nm

Inte

nsity

/ a.

u.

0Torr

0.05 1.80 3.75

16.07

vac.

Ex: 490nm

Fig. 12. The photoluminescence spectra of Cr-HMS (Si/Cr¼ 50) (top)

and CrS-1 (bottom) at 77 K and the effect of the addition of O2.

Scheme 3. The formation of charge transfer excited state with tetra-

hedrally coordinated chromium oxide moieties by UV and visible light

absorption and their radiative decay process (phosphorescence).

Cr6+

O O

O2-O2-

monomer species (open-type)

1.74 Å

1.57 Å

Cr6+

O O

O2-O2-

OH HO

monomer species (closed-type)

1.78 Å

O

Cr3+

O O

O O

O Cr3+

O

OO

O

agregated species

1.98 Å

(a) in meso-pore of HMS

(b) in micro-pore of silicalite-1

Fig. 13. The proposed local structures of the various Cr-oxide species.

0 50 100 150 2000

1

2

3

4

5

6

Amounts of added molecules /µ mol. g-cat-1

Φ0

Φ

(a) Excited at (a) 280 nm (b) 370 nm (c) 500 nm

(b)

(c)

Fig. 14. The quenching of the photoluminescence of the Cr-HMS (Si/

Cr¼ 50) by propane addition at 77 K.


vibration mode of Cr@O bonding. The spectrum indi-

cates that the energy separation between the bands for

the vibronic transition in the Cr@O bond is more clear

with Cr/HMS, while on the CrS-1 this band separationis very vague (Fig. 12). These results suggest the pres-

ence of some perturbation due to the neighboring sur-

face OH groups in the electronic state of the Cr@O

species included in the microporous silicalite-1 zeolite.

On the other hand, the mesopore of Cr/HMS is large

enough to avoid the formation of perturbation due to

near surface functional group. The coordination geom-

etry of the tetrahedrally coordinated Cr-oxide moietiesin CrS-1 might be distorted by the perturbation between

Cr@O bond and surface functional group of zeolite

cavities, while the open space of mesopore of HMS is

suitable to keep the isolated tetrahedrally coordinated

Cr-oxide moieties without perturbation. The proposed

local structures of the various Cr-oxide species are

shown in Fig. 13. Although the presence of this weak

interaction could be observed only by the analysis of the

photoluminescence, their detailed analysis is now in

progress.

The addition of propane or O2 onto the Cr-HMS led

to an efficient quenching of the phosphorescence in its

yield, its extent depending on the amount of gas added.

These results indicate that the charge transfer excited

state of the tetrahedrally coordinated isolated Cr-oxidemoieties, (Cr5þ–O�)�, easily interact with propane or O2

under UV light and visible light irradiations. As shown

in Fig. 14, with the excitation at the shorter wavelength,

more efficient quenching by the propane addition is

observed. These results indicate that the interaction

between the excited state of the Cr-oxide moieties and

the added propane is the stronger in the case of excita-

tion at the shorter wavelength than those at the longer

Fig. 16. Reaction time profiles of the photocatalytic NO decomposi-

tion on Cr-HMS and CrS-1 under UV light irradiation (k > 270 nm)

and visible light irradiation (k > 450 nm).


wavelength. The dependence of the efficiency in the

quenching on the excitation wavelength was also ob-

served with the addition of O2.

UV light irradiation (k > 270 nm) of the Cr-HMS inthe presence of propane and O2 led to the photocatalytic

oxidation of propane to produce acetone, acrolein, CO2/

CO etc. The Cr-HMS also shows photocatalytic reac-

tivity even under visible light irradiation (k > 450 nm).

The yields increase with the irradiation time. These re-

sults clearly indicate that Cr-HMS can absorb visible

light and act as an efficient photocatalyst under not only

UV light but also visible light irradiation [36,37,**38,39].As shown in Fig. 15, partial oxidation of propane

with a high selectivity for the production of oxygen

containing hydrocarbons such as acetone and acrolein

proceeds under visible light irradiation (k > 450 nm),

while further oxidation proceeds mainly under UV light

irradiation (k > 270 nm) to produce CO2 and CO. The

selectivity of partial oxidation production under visible

light irradiation (12% propane conversion) is higherthan those observed under UV light irradiation (26%

conversion) and even under UV light irradiation for the

shorter reaction time (11% conversion). These results

indicate that the tetrahedrally coordinated isolated Cr-

oxide moieties in HMS can exhibit the efficient photo-

catalytic reactivity for the oxidation of propane under

visible light irradiation with a high selectivity for the

partial oxidation of propane.UV light irradiation (k > 270 nm) of the Cr-HMS in

the presence of NO also led to the photocatalytic

decomposition of NO and the evolution of N2, N2O and

O2 in the gas phase at 275 K. The Cr-HMS also shows

photocatalytic reactivity even under visible light irradi-

ation (k > 450 nm). As shown in Fig. 16, the N2 yields

Fig. 15. The distribution of the photo-formed products in the pho-

tocatalytic oxidation of propane with the Cr-HMS (Si/Cr¼ 50) at 273

K under UV light irradiation (k > 270 nm) for (a) 0.5 h and (b) 2 h and

(c) visible light irradiation (k > 450 nm) for 2 h.

increase linearly to the irradiation time. Although the

reaction rate under the visible light irradiation is smaller

than under UV light irradiation, the selectivity for N2

formation (97%) under visible light irradiation is higher

than that of UV light irradiation (45%). These results

indicate that Cr-HMS can absorb visible light and act as

an efficient photocatalyst under not only UV light but

also visible light irradiations and especially Cr-HMS canbe useful to form N2 under visible light irradiation

[37,**38].

From the results obtained in the present study, a

schematic diagram for the energy levels of the Cr-oxide

moieties in a tetrahedrally coordinated structure has

been proposed as shown in Fig. 17, as well as the

2, NO

280nm 370nm 500nm 640nm

hυ'hυhυhυ

R

R

R

R

charge transfer excited state

(Cr5+-O-)

absorption absorptionabsorption

photoluminescence

ground state(Cr6+=O2-)

fast

fast

fast

reaction

energy transfer

R : Propane, O

(X) (Y) (Z)

Fig. 17. Energy diagram of Cr-oxide moieties in a tetrahedrally

coordinated structure and the mechanism of energy transfer, reaction,

and photoluminescence processes.


mechanisms for the energy transfer, the photocatalytic

reaction and the photoluminescence emission. In the

photocatalytic reactions, the selectivities under UV light

irradiation and visible light irradiation were different.The different excited states of the Cr-oxide moieties in

the tetrahedrally structure may interact with propane,

O2, and NO in different ways, causing such character-

istic reactivities.

4. Conclusions

Studies on the local structure and the dynamics of

photochemical reactivities of the well-defined transition

metal oxides (titanium, chromium oxides) incorporated

into the cavities and frameworks of various zeolites and

mesoporous molecular sieves clearly indicate that the

charge transfer excited state of these transition metal

oxide species, i.e., (Ti3þ–O�)�, and (Cr5þ–O�)� play a

vital role in the photoreactions of these oxide species.XAFS studies were useful to determine the coordination

geometry as a tetrahedral coordination and photolumi-

nescence investigations revealed that the efficient inter-

action of the charge transfer photoexcited complexes of

these tetrahedrally coordinated oxides, (Ti3þ–O�)� and

(Cr5þ–O�)�, with gaseous reactants, such as NO, CO2,

H2O, alkanes and O2 plays a significant role in the

photocatalytic reactions. The extremely highly localizedcharge separation realized on these excited tetrahedral

transition metal oxides leads to unique photocatalytic

properties quite different from semiconductor type

photocatalysts (TiO2 etc.) in which the photoformed

holes and electrons rapidly separate from each other and

move to relatively large distances. The present results

have demonstrated that the unique physicochemical

properties of various zeolites and mesoporous materialssuch as the pore size, the channel structural dimensions,

and distribution of ion-exchangeable sites are useful to

control the dispersion and the local structure of the

anchored metal oxides.

References

The papers of particular interest have been high-

lighted as:

* of special interest;

** of outstanding interest.

[1] Thomas JM. Design, synthesis and in situ characterisation of

new solid catalysts. Angew Chem Int Ed 1999;38:3588–628.

[2] Corma A. From microporous to mesoporous molecular sieve

materials and their use in catalysis. Chem Rev 1997;97:2373–

419.

[3] Notari B. Microporous crystalline titanium silicalites. Adv

Catal 1996;41:253–334.

[**4] Anpo M, Che M. Applications of photoluminescence tech-

niques to the characterization of solid surfaces in relation to

adsorption, catalysis and photocatalysis. Adv Catal 2000;

44:119–257.

[5] Anpo M, editor. Photofunctional zeolites. New York: NOVA

Publishers Inc.; 2000.

[6] Anpo M, editor. Surface photochemistry. London: J. Wiley &

Sons, Inc.; 1996.

[7] Anpo M, Kubokawa Y. Photo-induced and photocatalytic

reactions on supported metal oxide catalysts: Excited states of

oxides and reaction intermediates. Res Chem Intermed

1987;8:105–24.

[8] Anpo M, Yamashita H, Zhang SG. Photo-induced surface

chemistry. Curr Opinion Solid State Mater Sci 1996;1:630–8.

[9] Inui T, Medhanavyn D, Praserthdam P, Fukuda K, Ukawa T,

Sakamoto A, et al. Methanol conversion to hydrocarbons on

novel vanadosilicate catalysts. Appl Catal 1985;18:311–24.

[10] Patterson HH, Cheng J, Despres S, Sunamoto M, Anpo M.

Relationship between the geometry of the excited state of vana-

dium oxides anchored onto silica and their photoreactivity

toward carbon monoxide molecules. J Phys Chem 1991;95:

8813–8.

[11] Rigutto MS, VanBekkum T. Synthesis and characterization of

a thermally stable vanadium-containing silicalite. Appl Catal

1991;68:L1–7.

[12] Centi G, Perathoner S, Trifir�o F, Aboukais A, Aiss CF, Gueltin

M. Physicochemical characterization of V-silicalite. J Phys

Chem 1992;96:2617–29.

[13] Dzwigaj S, Matsuoka M, Franck R, Anpo M, Che M. Probing

different kinds of vanadium species in the VSi-beta zeolite by

diffuse reflectance UV-visible and photoluminescence spectros-

copies. J Phys Chem B 1998;102:6309–12.

[*14] Anpo M, Zhang SG, Higashimoto S, Matsuoka M, Yamashita

H, Ichihashi Y, et al. Characterization of the local structure of

the vanadium silicalite (VS-2) catalyst and its photocatalytic

reactivity for the decomposition of NO into N2 and O2. J Phys

Chem B 1999;103:9295–301.

[15] Higashimoto S, MatsuokaM, Yamashita H, AnpoM, Kitao O,

Hidaka H, et al. Effect of the Si/Al Ratio on the local structure

of V-oxide/ZSM-5 catalysts prepared by solid-state reaction

and their photocatalytic reactivity for the decomposition of NO

in the absence and presence of propane. J Phys Chem B

2000;104: 10288–92.

[16] Higashimoto S, Tsumura R, Matsuoka M, Yamashita H, Che

M, Anpo M. Local structures of active sites on Mo-MCM-41

mesoporous molecular sieves and their photocatalytic reactivity

for the decomposition of NOx. Stud Surf Sci Catal 2001;

140:315–21.

[17] Anpo M, Matsuoka M, Shioya Y, Yamashita H, Giamello E,

Morterra C, et al. The preparation of the Cu+/ZSM-5 catalyst

and its interaction with NO, and photocatalytic decomposition

of NO into N2 and O2 on the catalyst at 275 K-in situ

photoluminescence, ESR and FT-IR investigation. J Phys

Chem 1994;98:5744–50.

[*18] Yamashita H, Matsuoka M, Tsuji K, Shioya Y, Anpo M. In

situ XAFS, photoluminescence, and IR investigations of copper

ions included within various kinds of zeolites––The structure of

Cu (I) ions and their interaction with CO molecules. J Phys

Chem 1996;100:397–402.

[19] Ju WS, Matsuoka M, Iino K, Yamashita H, Anpo M. The local

structures of silver (I) ion catalysts anchored within zeolite

cavities and their photocatalytic reactivities for the elimination

of N2O into N2 and O2. J Phys Chem B 2003;108:2128–33.

[20] MatsuokaM, Matsuda E, Tsuji K, Yamashita H, AnpoM. The

photocatalytic decomposition of nitric oxide on the Ag+/ZSM-

5 catalyst prepared by the ion-exchange method. J Mol Catal

1996;107:399–403.

[*21] Bordiga S, Coluccia S, Lamberti C, Marchese L, Zecchina A,

Boscherini F, et al. XAFS study of Ti-silicalite: structure of


framework Ti (IV) in the presence and absence of reactive

molecules (H2O, NH3) and comparison with ultraviolet-visible

and IR results. J Phys Chem 1994;98:4125–32.

[22] Gianotti E, Yoshida H, Dellarocca V, Marchese L, Martra G,

Coluccia S. Photoluminescence study of mesoporous MCM-41

and Ti-grafted MCM-41. Res Chem Intermed 2003;29:681–9.

[23] Zhanpeisov NU, Matsuoka M, Mishima H, Yamashita H,

Anpo M. Cluster quantum chemistry ab initio study on the

interaction of NO molecules with highly dispersed titanium

oxides incorporated into silicalite and zeolites. J Phys Chem B

1998;102:6915–20.

[24] Ichihashi Y, Yamashita H, Anpo M, Souma Y, Matsumura Y.

Photoluminescence properties of tetrahedral titanium oxide

species formed in zeolitic materials. Catal Lett 1998;53:107–9.

[25] Thomas JM. The high-resolution structural characterisation

and the rational design of inorganic solid catalysts. Farad Dis

1996;105:1–31.

[26] Sankar G, Thomas JM. In situ combined X-ray absorption

spectroscopic and X-ray diffractometric studies of solid cata-

lysts. Top Catal 1999;8:1–21.

[**27] Thomas JM, Sankar G. The role of synchrotron-based studies

in the elucidation and design of active sites in titanium-silica

epoxidation catalysts. Acc Chem Res 2001;34:571–81.

[28] Yamashita H, Ikeue K, Anpo M. Photocatalytic Reduction of

CO2 with H2O on Various Titanium Oxide Catalysts. ACS

Symposium Series 809, Book on CO2 Conversion and Utiliza-

tion. ACS; Washington: 2002. pp. 330–343.

[*29] Anpo M, Yamashita H, Ichihashi Y, Fujii Y, Honda M.

Photocatalytic reduction of CO2 with H2O on titanium oxides

anchored within micropores of zeolites: effects of the structure

of the active sites and the addition of Pt. J Phys Chem B

1997;101:2632–6.

[30] Yamashita H, Fuji Y, Ichihashi Y, Zhang SG, Ikeue K, Park

DR, et al. Selective formation of CH3OH in the photocatalytic

reduction of CO2 with H2O on titanium oxides highly dispersed

within zeolites andmolecular sieves. Catal Today 1998;45:221–7.

[*31] Ikeue K, Yamashita H, Takewaki T, Anpo M. Photocatalytic

reactions of CO2 with H2O on Ti-Beta zeolite photocatalysts:

effect of the hydrophobic and hydrophilic properties. J Phys

Chem B 2001;105:8350–5.

[32] Yamashita H, Ikeue K, Takewaki T, Anpo M. In situ XAFS

studies on the effects of the hydrophobic-hydrophilic properties

of Ti-Beta zeolites in the photocatalytic reduction of CO2 with

H2O. Topics Catal 2002;18:95–100.

[*33] Yamashita H, Ichihashi Y, Anpo M, Hashimoto M, Louis C,

Che M. Photocatalytic decomposition of NO at 275 K on

titanium oxides included within Y-zeolites cavities: the structure

and role of the active Sites. J Phys Chem 1996;100:16041–4.

[34] Yamashita H, Zhang SG, Ichihashi Y, Matsumura Y, Souma

S, Tatsumi T, et al. Photocatalytic decomposition of NO at 275

K on titanium oxide catalysts anchored within zeolite cavities

and frameworks. Appl Surf Sci 1997;121:305–9.

[35] Zhang J, Minagawa M, Ayusawa T, Natarajan S, Yamashita

H, Matsuoka M, et al. In situ investigation of the photocat-

alytic decomposition of NO on the Ti-HMS under flow and

closed reaction systems. J Phys Chem 2000;104:11501–5.

[36] Yamashita H, Ariyuki M, Higashimoto S, Zhang SG, Chang

JS, Park SE, et al. Characterization and photocatalytic reac-

tivities of Cr-HMS mesoporous molecular sieves. J Synchrotron

Rad 1999;6:453–6.

[37] Yamashita H, Yoshizawa K, Ariyuki M, Higashimoto S, Anpo

M. photocatalytic ethylene polymerization on chromium con-

taining mesoporous molecular sieves. Stud Surf Sci Catal

2002;141:495–502.

[**38] Yamashita H, Yoshizawa K, Ariyuki M, Higashimoto S, Che

M, Anpo M. Photocatalytic reactions on chromium containing

mesoporous silica molecular sieves (Cr-HMS) under visible

light irradiation: decomposition of NO and partial oxidation of

propane. Chem Commun 2001:435–6.

[39] Yamashita H, Ohshiro S, Kida K, Yoshizawa K, Anpo M.

Visible light responsive photocatalytic reaction on tetrahedrally

coordinated chromium oxide moieties loaded on ZSM-5

zeolites and HMS mesoporous silica: partial oxidation of

propane. Res Chem Intermed, 2003;29:881–90.

[40] Yamashita H, Ichihashi Y, Harada M, Stewart G, Fox MA,

Anpo M. Photocatalytic degradation of 1-octanol on anchored

titanium oxide and on TiO2 powder catalysts. J Catal 1996;158:

97–101.

[41] Yamashita H, Kawasaki S, Ichihashi Y, Harada M, Anpo M,

Stewart G, et al. Characterization of titanium-silicon binary

oxide catalysts prepared by the sol–gel method and their

photocatalytic reactivity for the liquid phase oxidation of 1-

octanol. J Phys Chem B 1998;102:5870–5.

[42] Yamashita H, Honda M, Harada M, Ichihashi Y, Anpo M,

Hirao T, et al. Preparation of titanium oxide photocatalysts

anchored on porous silica glass by a metal ion-implantation

method and their photocatalytic reactivities for the degradation

of 2-propanol diluted in water. J Phys Chem B 1998;102:10707–

11.

[43] Anpo M, Kondo M, Coluccia S, Louis C, Che M. Application

of dynamic photoluminescence spectroscopy to the study of the

active surface sites on supported Mo/SiO2 catalysts. Features of

anchored and impregnated catalysts. J Am Chem Soc 1989;111:

8791–9.

[44] Anpo M, Chiba K. Photocatalytic reduction of CO2 on

anchored titanium oxide catalyst. J Mol Catal 1992;74:207–12.

[45] Yamashita H, Nishiguchi H, Kamada N, Anpo M, Teraoka H,

Ehara S, et al. Photocatalytic reduction of CO2 with H2O on

TiO2 and Cu/TiO2 catalysts. Res Chem Intermed 1994;20:815–

23.

[46] Zhang W, Tanev PT, Pimmavaia TJ. Catalytic hydroxylation of

benzene over transition-metal substituted hexagonal mesopor-

ous silicas. J Chem Soc Chem Commun 1996:979–80.

[47] Wu P, Tatsumi T, Komatsu T, Yashima T. A Novel titanos-

ilicate with MWW Structure. I. Hydrothermal synthesis,

elimination of extra framework titanium, and characterizations.

J Phys Chem B 2001;105:2897–905.

[48] Farges F, Brown GE, Rehr JJ. Coordination chemistry of Ti

(IV) in silicate glasses and melts. 1. XAFS study of titanium

coordination in oxide model compounds. Geochim Cosmochim

Acta 1996;60:3023–38.

[49] Anpo M, Tanahashi I, Kubokawa Y. Photoreducibility of

supported metal oxides and lifetimes of their excited triplet

states. J Phys Chem 1982;86:1–3.

[50] Weckuysen BM, Schoonheydt RA, Mabbs DE, Collison D.

Electron paramagnetic resonance of heterogeneous chromium

catalysts. J Chem Soc Farad Trans 1996;92:2431–6.

[51] Hazenkamp MF, Blasse G. A luminescence spectroscopy study

on supported vanadium and chromium oxide catalysts. J Phys

Chem 1992;96:3442–6.

Local structures and photocatalytic reactivities of the titanium oxide and chromium oxide species incorporated within micro- and mesoporous zeolite materials: XAFS and photoluminescence

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